1. Field of the Invention
The present invention relates to electrical lighting and, in particular, to circuits for operating electric lights.
2. Description of the Prior Art
Continuous maintainance of proper illumination is critical to the effective performance of surgical procedures. To ensure that illumination having an appropriate pattern is available during the performance of the procedure, surgical lights are often employed which are capable of providing two or more illumination patterns. The surgical lights include light sources which are energized either individually or together in any one of at least two combinations to make available more than one illumination pattern. Often, the surgical light is provided with different types of light sources each of which can produce a different light pattern. Each source can be either one of several different types of lamps or one filament of a lamp having multiple filaments. Accordingly, switching energizing power between lamps or filaments changes the illumination pattern provided by the light.
Usually, the surgical light includes a lighting control system which permits manual switching of energizing power among the light sources of the light to change the illumination pattern created by the light. Further, many surgical lights reduce the risk of loss of illumination during performance of a surgical procedure by including in the surgical light a lighting control system which permits manual switching of energizing power from a failed source--a source which no longer provides adequate illumination--to a backup source, which may be one of the sources which provides an illumination pattern different from those provided by the energized sources and by the failed source.
Requiring manual switching of energizing power from a failed light source to an operable source presents the obvious disadvantage of temporary loss of adequate illumination between the times of failure of the failed source and energization of the backup source.
Conventional surgical lights require manual actuation of a switch at the light head, the housing which contains and supports the light sources of the surgical light, to switch energizing power among light sources and achieve a change in the illumination pattern. If a person other than one of the members of the normal surgical team is provided to operate the surgical light at the light head, the already congested surgical area becomes more congested. If a member of the normal surgical team is assigned the duty of operating the surgical light, that member's attention is periodically diverted from that member's normal duties to operate the light, with the possible degradation of overall performance by that member. Therefore, it would appear to be desirable to permit manual switching of energizing power from one source to another by providing a controller mounted at a location remote from the light head to permit its operation by a person other than a member of the normal surgical team and without adding to the congestion of the surgical area. To applicant's knowledge, with one exception, a lighting control system having such a remotely mounted controller has not been provided due to the complexity involved in providing the system with the capability of switching energizing power among light sources using an acceptably low number of electrical conductors between the remote controller and the light head. The exception is the system described in Application for U.S. Letters Patent Ser. No. 362,117, filed Mar. 26, 1982, and owned by the assignee of the present application, which switches energizing power from one light source to another by reversing the polarity of the voltage applied by the controller to the light head of the surgical lamp.
Therefore, there exists a need for a circuit which automatically switches energizing power among light sources when adequate illumination is lost. There exists a further need for a circuit which automatically de-energizes a failed light source and energizes an operable source upon failure of the failed source. There exists a further need for a circuit which monitors the operability of a backup light source and provides an indication that a backup source has failed to ensure that a backup source is available upon failure of an energized source. Further, there is a need for a circuit which permits provision of a lighting control system having a controller mounted at a location remote from the light head of a surgical light which communicates with the light head over only two electrical conductors, without requiring a reversal of the voltage applied to the light by the controller.